1. Field of the Invention
This invention relates to acoustic imaging of mammalian airway morphology and more particularly concerns noninvasively obtaining a signal representative of the cross-sectional area of an airway (e.g., oral, nasal, or pulmonary) of a subject (e.g., a person or an animal) using electroacoustical transducers.
2. Brief Description of Related Art
A one-dimensional image of the cross-sectional area of an airway as a function of axial position along the airway may be determined from acoustic reflections measured by a single electro-acoustic transducer placed in a position remote from the airway opening. This image is referred to as an area-distance function and is represented by A(x) where x is the axial position along the airway.
Knowledge of the area-distance function, A(x), is useful for example in the diagnosis of mammalian pathologies associated with oral airways, larynx, pulmonary airways, and nasal airways. These pathologies include but are not limited to obstructive sleep apnea, asthma, obstructive pulmonary disease, tracheal stenosis, and nasal septum deviation.
Accurate information about the area-distance function is also useful in the study of airway growth and its disruption and sequelae of bronchopulmonary dysplasia in children.
One approach to using a single electro-acoustic transducer in acoustic imaging is described in U.S. Pat. No. 4,326,416 granted Apr. 27, 1982, to Jeffrey J. Fredberg entitled ACOUSTIC PULSE RESPONSE MEASURING. In all of the single-transducer approaches described previously, a hidden constraint pertains. The associated theories assume implicitly that once propagating to the left within the apparatus, acoustic returns encounter no reflection sites within the wave-tube apparatus itself, which is assumed to be a reflectionless transmission line; there must be no secondary rightward travelling waves and no acoustic reverberation within the wave tube. However, since the loudspeaker is an unavoidable and major reflection site for the wave travelling to the left, the distance separating the loudspeaker from the receiving transducer must be greater than the maximum airway penetration depth of interest; this ensures that secondary reflections from the speaker arrive at the receiving transducer only after data acquisition has been completed. As a result of this distance constraint, imaging instruments for airway imaging previously described in the literature are 1 to 2 meters or more in length (Brooks et al., Reproducibility and Accuracy of Airway Area by Acoustic Reflection, J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 57(3): 777-787, 1984; D'Urzo et al., Effect of CO.sub.2 Concentrations on Acoustic Inferences of Airway Area, J. Appl. Physiol. 60:398-401, 1986); and some are as long as 5 meters (Fredberg J. J. et al., Airway Area From Acoustic Reflections Measured at the Mouth, J. Appl. Physiol.: Respirat. Environ. Exercise. Physiol. 48(5): 749-758, 1980).
A two-transducer approach is described in a paper of M. R. Schroeder entitled "Determination of the Geometry of the Human Vocal Tract by Acoustic Measurements" in J. Acoust. Soc Am. 41(4), 1002-10 (1967). However, as with the single-microphone approach, Schroeder's two-microphone method was never embodied successfully into a small, compact, hand holdable light-weight working apparatus and in fact never achieved airway reconstructions from human or animal airways.
The present invention is based upon a new two-transducer method and a new associated theory that permits practical application of airway reconstructions by acoustic reflections. Because this theory explicitly incorporates reverberation within the wave tube and does not demand non-overlapping time windows of incident and reflected waves, it removes the distance constraint described above, permitting placement of the loudspeaker or launching transducer close to the receiving transducers.
As such, the new theory allows fabrication of a practical miniature apparatus whose overall length is only a few centimeters rather than meters. The apparatus can image the respiratory tract of mammalian subjects, including the nasal oral and pulmonary cavities.